The common brushtail possum (Trichosurus vulpecula) is one of few invasive marsupial species. It is the most widespread mammal species in Australia and its range has been artificially extended into New Zealand.

The common brushtail possum has become established throughout almost all of New Zealand's North and South Islands. They are also present on Stewart Island and Chatham Island, but have been eradicated from most other large islands. Two-thirds of the possums are present in North Island. Average possum density in New Zaland stands at around 4 animals per hectare but can reach 25 animals per hectare. In the early 2000s, total population estimates ranged from 50 million to 70 million. [2]

Common brustail possums were first introduced to New Zealand in 1837 but did not become established. Reintroduction in 1858 occurred in Southland to set up populations to be farmed for their fur. The first few releases were private but acclimitisation societies took over the role from 1870. Thirty-six more batches were imported from then until 1926 and by 1930, possums had been released in 450 locations around New Zealand. Even after the release of possums was finally prohibited in 1921, trappers continued to release animals to hunt. [2]

It is estimated that by 1950, possums had spread accross 54% of the mainland. By 1963 thisfigure had increased to 84% and by 1980 it had increased to 91%. [2]

The common brushtail possums of New Zealand have had a sizeable effect on the structure and extent of forests. Many valleys lost more than half of their canopy trees within 20 years of the arrival of possums. Possums will feed on and therefore remove their preferred tree species and will change the structure of the forest. The possums will eat fresh shoots, slowing ther growth of new trees, and will also feed on flowers, so that seeding can't occur. [2]

Additionally, common brushtail possums eat bird eggs and hatchlings. Of 19 nests belonging to the endangered kokako which were monitored over four years, 4 of the nests were attacked by possums.. Even some adult birds are eaten. The possums also compete with birds for food, as well as driving kiwis out of their dens. Bats are also eaten and outcompeted for food. [2]

Invertebrates, included endangered species, are also eaten. It has been found that one possum can eat 60 large Powelliphanta snails in a single night. [2]

Also, the common brushtail possums are carriers of bovine tuberculosis, with an average of 2% infected and peaks of 40% in some areas. The possums also eat pasture. Costs to New Zealand's agricultural sector are around $35 million dollars per year. [2]

Hunting of common brushtail possums in New Zealand was regulated until their protection was removed in 1946, when the possums were first officialy recognised as invasive for the first time. [2]

A bounty scheme was run from 1951 to 1961, with payouts of two shillings and sixpence for each possum killed and brought in. Over 8 million bounties were paid out. However, three quarters of the animals were collected in farms or collected as roadkill or taken from open, accessible places. Possum populations continued to grow in the forests. [2]

Currently, mosts possums are killed using poisons and lethal traps. Such methods have removed possums from many of New Zealand's islands, including Rangitoto, Motutapu, Kapiti, Codfish, Whanganui and Tarakaipa. On Kapiti island alone, 19,612 animals were killed between 1980 and 1986. Bird counts from 1982 to 1988 doubled on the island. [2]

The six main poisons used to reduce possum populations are phosphorus, cholecalciferol, pindone, cyanide, brodifacoum and sodium monofluoroacetate, more commonly known as 1080. The lattermost of these poisons, 1080, is most commonly used, Its use is contraversial, as it also affects native species, although it does dissolve and dilute quickly in water and soil. [2]

Common brushtail possums are sometimes trapped for their fur. Possum furs constitute 1% of the fur industry and many materials from the animals, including their pelts for leather and their meat for food, is used. [2] Noting that this industry was the reason for the introduction of the animals to New Zealand originally, this control method is only minor and may be flawed.

The use of reproductive vaccines is being investiagted and evaluated. Trials have shown that possum breeding has reduced by 70 - 80% within treated possums. The vaccine, which would be given to possums by baiting, have not yet been used in the wild. [2]